The present invention relates generally to shot peening, and, more specifically, to laser shock peening.
In conventional shot peening, small balls are fired against the surface of a metallic workpiece or target to create plastic deformation and a corresponding residual compressive stress. The residual stress improves the useful fatigue life of the workpiece when used in a high stress application.
Laser shock peening (LSP) is being developed to provide improvements in forming the residual compressive stress in a workpiece. A laser is operated in a pulse mode for directing laser pulses against a workpiece surface, which surface typically has a light absorbing ablative coating confined by a thin, optically transparent, inertial confinement layer of water, for example. The laser pulse vaporizes the coating in a small explosion to form a high temperature plasma that is confined by the water, developing an instantaneous pressure pulse or shock wave that plastically deforms the workpiece surface to generate residual compressive stress.
A significant control parameter in this process is the fluence of the laser beam that is defined as the pulse energy per unit area of the beam. Fluence at the target must exceed a threshold value for effecting laser shock peening, but fluence in the resonator or oscillator producing the laser beam must be low to prevent optical damage.
This control may be accomplished by using a low energy oscillator followed in turn by several increasingly large aperture rod amplifiers in which pulse energy is increased while the fluence is maintained below the damage threshold, protecting the laser equipment from optical damage. However, this configuration requires many laser heads or lasing gain media, optical pumping flash lamps, power supplies, and related equipment that increases the complexity of the system and the system's susceptibility to failure during operation.
Another useful laser system having few components is disclosed in U.S. Pat. No. 5,730,811-Azad et al. In this configuration, a large Q-switched, cavity-dumped, laser oscillator produces a significant fraction of the required energy for laser shock peening in a single-head resonator, with an additional amplifier cooperating therewith. This high energy laser requires suitably large aperture optical elements and a Pockels cell for effecting the Q-switching and cavity-dumping. The large elements reduce the fluence in the oscillator for preventing thermal damage thereto from the high energy laser beam created therein.
Another significant control parameter is the intensity or irradiance, defined as the fluence divided by the pulse duration. Since the desired residual stress distribution of a workpiece covers a finite area, and the LSP processing requires a fluence threshold that depends upon the workpiece material, a minimum energy per laser pulse is required. Since the treatment of production parts entails processing areas much larger than a single laser spot, the throughput of the process is limited by the average power and repetition rate of the source laser system.
The operating laser parameters contributing to the intensity control parameter including pulse energy, fluence, duration, intensity, and repetition rate, along with laser system size and efficiency are interrelated in effecting laser shock peening in a commercially viable system. Particularly problematic in presently known laser shock peening are the high energy requirements such as 50 Joules per pulse, 200 Joules/square centimeter, and 20 nanosecond pulse duration, that require large, complex, and expensive laser systems operating close to their damage threshold design limits, having correspondingly limited throughput capability and efficiency, and correspondingly high maintenance requirements.
Accordingly, it is desired to effect laser shock peening at lower pulse energy and increased pulse repetition rates for improving efficiency and throughput, and lowering maintenance cost and down-time.